An enzyme is designed to bind most tightly to a substrate when it is in the transition state of the reaction which the enzyme catalyses. The consequent reduction of the activation energy of the reaction constitutes the catalytic mechanism. The energetic contributions of different features of the interaction can only be crudely assessed, but they are dominated by entropically driven effects. The binding site of trypsin orients the substrate so that the reacting groups are correctly placed for reaction to occur. Apart from two side chains which take part in chemical steps of the reaction, the enzyme behaves almost as a rigid body. The full binding interactions are only developed when the substrate is in an intermediate stage of the reaction. The tightly bound complexes of trypsin with protein trypsin inhibitors have proved amenable to structural analysis. Enzyme inhibitor interactions, which account for almost 80 kJ mol -1 of interaction energy, are known fairly accurately. The similarity of the two known trypsin inhibitor structures, close to the primary binding site, indicates a high specificity, even for this simple interaction. In cases where no large conformational changes occur the specificity of an enzyme should be predictable from accurate knowledge of its tertiary structure.